Common ores that include metal values such as cobalt, nickel, silver, iron, copper and mercury are accompanied by arsenic and sulphur. Deposits such as those occurring in the vicinity of Cobalt, Ontario, Canada serve as examples.
The Cobalt, Ontario, Canada region has produced over 400 million troy ounces of silver and unrecorded but very substantial amounts of cobalt. During the early years the main emphasis was on silver extraction and recovery, those procedures involving mainly gravity concentration, amalgamation and cyanidation.
The increase in the value of cobalt and silver in recent years has focussed attention upon available feedstocks containing those metal values. Such feedstocks include stockpiled leach residues, flotation concentrates, silver gravity concentrates, cobalt gravity concentrates and refinery speiss wherein cobalt may be present in a range of about 2%-20%, nickel in a range of from 1%-5%, silver in a range of from about 40 troy ounces per short ton to 2,000 troy ounces per short ton and containing iron in a range of from about 8%-20%, arsenic in a range of from about 10%-40%, sulphur in a range of from about 5%-20% and mercury up to 1/10th of 1%.
Substantially complete removal of the arsenic and sulphur from the concentrates, residues or speiss is necessary at the outset for economic recovery of the cobalt, nickel and silver values from such sources.
Moreover, not only is such initial separation of arsenic and sulphur essential in any method to be adopted, but such method should contemplate the conversion of those elements, arsenic and sulphur, into compounds having an economic value, a marketable commodity, which is not only of importance to the commercial considerations underlying the project, but will likely satisfy any environmental conditions that apply to any waste or effluent from the system to be introduced.
Known recovery methods include roasting of such feedstock to initially remove arsenic and sulphur followed by leaching and other steps. Those approaches are inefficient as well as objectionable both from the point of view of the character of the product generated and the high cost of the controls that must be exercised to meet operational as well as environmental standards. Accordingly, those alternatives are not attractive under prevailing conditions.
On the other hand, certain proposals for the treatment of such feedstock advocate oxidative pressure leaches involving ferric chloride, ferric sulphate and basic solutions for the separation and recovery of the constituents, with the last mentioned alternative having made the greatest impact during an evaluation of same.
More particularly, one such proposal recommending an oxidative pressure leach commences with a pretreatment of feedstock of the type under consideration, with a basic solution, particularly sodium hydroxide which is heated and then subjected to elevated pressures and temperatures in the presence of finely dispersed air. This proposal appears in U.S. Pat. No. 2,951,741. Initial removal of arsenic and sulphur from the concentrate containing cobalt, nickel and silver by oxidizing the finely divided ore constituents in the circumstances outlined, is accomplished by converting the arsenic and sulphur into soluble alkaline metal salts leaving the primary metallic values in an insoluble elemental state or as insoluble oxides.
An investigation of the approach disclosed by U.S. Pat. No. 2,951,741 has revealed that the directions are inadequate and incomplete and particularly fail to indicate or prescribe conditions that would ensure substantially complete separation of the arsenic and sulphur in the pretreatment stage or within a reasonable period of time; nor is efficient effective recovery of arsenic and sulphur in the form of marketable products given detailed consideration such that it could be said to merit exploitation as prescribed.
Finally, it is essential that any method adopted and the particular processes introduced should aim at recovering substantially all of the metal values in that the economic feasibility of the project is a controlling factor in the circumstances.